1. Field of the Invention
The present invention relates to a method of measuring a voltage on, for example, an integrated circuit with an electron beam apparatus, to thereby diagnose the integrated circuit.
2. Description of the Related Art
FIG. 1 shows essential part of a conventional electron beam apparatus.
In the figure, numeral 1 denotes a lens barrel, 2 an electron gun, 3 an electron beam emitted from the electron gun 2, 4 an electron lens, 5 a blanker for shaping the electron beam 3 into a pulse strobe beam 6, 7 a deflector coil for deflecting the strobe beam 6, 8 a sample chamber, 9 a sample LSI to be measured, 10 secondary electrons produced by the sample LSI 9, 11 an energy analyzer having an analyzer grid 12 for producing a decelerating electric field for controlling a passage of the secondary electrons 10, and 13 a secondary electron detector. In this electron beam apparatus, the voltage of a signal to be measured on the sample LSI 9 is given as an analytic voltage that provides a secondary electron signal level that is equal to a slice level set on an analytic-voltage-to-secondary-electron-signal-level-characteristic curve, i.e., a so-called S curve. To find the voltage, a closed loop method is employed.
The closed loop method will be explained with reference to FIGS. 2 and 3.
In FIG. 2, a slice level SL is set on an S curve. Around the slice level SL, the S curve has an inclination of .beta.(=.delta.S/.delta.V). An inverse number of the inclination .beta. is a convergence factor .alpha.(=-.delta.V/.delta.S).
In FIG. 3, an initial analytic voltage VR0 is applied to the analyzer grid 12, and a secondary electron signal is sampled. The level of the sampled secondary electron signal is S0, and the analytic voltage is updated by .alpha.(S0-SL). The updated voltage of VR1=VR0+.alpha.(S0-SL) is applied to the analyzer grid 12, and a secondary electron signal is sampled. The level of the sampled secondary electron signal is S1, and the analytic voltage is updated by .alpha.(S1-SL). The updated voltage of VR2=VR1+.alpha.(S1-SL) is applied to the analyzer grid 12.
In this way, the analytic voltage is successively updated. When the absolute value of an analytic voltage updating quantity becomes smaller than a given value, an analytic voltage that may provide the slice level SL is measured several times, and the measured voltages are added and averaged to provide a measured voltage of an objective signal.
As shown in FIG. 4, the S curve of the electron beam apparatus changes due to contamination during measurement and beam current fluctuations caused by changes in a beam axis, thereby changing the convergence factor .alpha.. In spite of this fact, the conventional technique computes the convergence factor .alpha. of the electron beam apparatus only once at the start of measurement and never updates it during the measurement, thereby deteriorating the accuracy of measurement.
In addition, the conventional technique often scans a phase of measurement synchronously with, for example, a counter circuit. This may cause an error in a detected secondary electron signal level because of the influence of an insulation film disposed over wiring of an IC whose voltage is to be measured. To avoid this, the conventional technique employs a method of scanning a phase of measurement at random.
FIG. 5 shows an example of a signal to be measured. Scanning a phase of measurement at random may provide analytic voltages, i.e., measured voltages as shown in FIG. 6.
According to this method, the number of sampling strobes at each phase may be 100 to 500. With a sampling rate of 10 MHz, a sampling time at each phase will be about 10 to 50 .mu.s.
The analytic voltage, however, must not be changed too rapidly because this may increase loads on the analyzer grid 12 and peripheral circuits. After the analytic voltage is changed, a settling time of about 20 .mu.s is needed. This settling time is nearly equal to the sampling time. Accordingly, a secondary electron signal is sampled not with a specified analytic voltage but with an equivalent analytic voltage Veff expressed as follows: ##EQU1## where V0 is an analytic voltage of the last measurement, V1 is an analytic voltage of this time, .tau.s is a time necessary for sampling a secondary electron signal at a given phase, and .tau.v is a settling time of the analytic voltage (FIG. 7). Accordingly, the analytic voltage of this time involves an error proportional to an analytic voltage change (V0-V1). Around a slice level, the analytic voltage is proportional to a secondary electron signal level. Accordingly, the error proportional to the change (V0-V1) is superimposed on the secondary electron signal. This error is observed as noise on a measured voltage. To avoid the noise, the conventional technique puts a settling time of about 10 to 20 .mu.s after the analytic voltage is changed, and after the settling time, starts sampling a secondary electron signal. This raises a problem of elongating measuring time.